Femtocells (also referred to as “Femtos” in this specification) are low power, low cost, generally user-deployed base stations that are able to provide exceptional cellular service in residential or enterprise environments, with a typical coverage range of tens of meters. They have extensive auto-configuration and self-optimization capabilities to enable a simple ‘plug and play’ deployment, and are designed to automatically integrate themselves in an existing macrocellular network. Femtocells typically use the customer's broadband internet connection (e.g. Digital Subscriber Line (DSL), cable, etc.) as backhaul.
A typical femtocell architecture is shown in FIG. 1, where the ‘Home cell’ shown on the right hand side is a femtocell and is connected to a Home cell, or Femtocell, Controller/Gateway. In the particular architecture illustrated, elements on the left hand side of the figure are included in a Universal Mobile Telecommunications System (UMTS) macrocellular network, and include a base station known as a NodeB, a Radio Network Controller (RNC), Mobile Switching Centre (MSC), Serving GPRS Support Node (SGSN), and Gateway GPRS Support Node (GGSN). Usually there is a connection to a public switched telephone network (PSTN).
In UMTS femtocell arrangements, the number of primary scrambling codes (PSC) which may be allocated to a Femto is limited. The number that can be allocated is typically between 6 and 16. The selection of a primary scrambling code is typically auto-configured by the femtocells themselves based on measurements of other primary scrambling codes used in their vicinity. Therefore, in typical femtocell deployments, these scrambling codes will need to be heavily re-used. With a high level of PSC re-use, it is likely that a user equipment (UE) served by a macro cell, or a femtocell, that measures femtocells, will report the same PSC from a number of different cells. As currently defined in 3rd Generation Partnership Project (3GPP) standards, a UE does not provide the Cell ID in its UE measurement report. Therefore, it is difficult for the target network to use measurements supplied by the UE to automatically determine the actual target cell when performing either macro to Femto, or Femto to Femto Handover procedures.
When handling an incoming handover request from a macro cell, the Home Cell Gateway must be able to determine to which femtocell the UE is handing over. Efficient determination and subsequent reporting of the target cell for a handover procedure is an important requirement in maintaining a high quality of service to the UEs, since any interruption to an on-going session on the UE needs to be minimised. However existing 3GPP standard messages do not provide a mechanism for the source cell to uniquely identify the target cell, as the UE will not report the Cell ID of the target cell in measurement reports to the source cell. The UE is able to report the PSC of the target cell. However, when PSCs are re-used, the Gateway cannot rely on the reported value of the target cell PSC to uniquely identify it. Similarly for Femto to Femto Handovers, the source femtocell also must be able to determine the target femtocell for the handover to complete effectively, but again, the target cell may not be uniquely identified where there is a high level of PSC re-use.
Where the Gateway, or the source Femto, is not able to uniquely determine the target femtocell from the information provided by the UE in the source cell, it makes it difficult for the Gateway or source Femto to decide to which femtocell the Handover Request should be forwarded. This is especially important when considering a typical Enterprise/Office building which is likely to have many femtocells deployed.
One possible approach is for the Femto Gateway to make a guess about the correct target femtocell using the limited information provided as part of the handover request. This solution is likely to lead to an increase in the duration of the handover procedure, which may in turn impact the end user's on-going call, for example, leading to packet loss or even call drop.
Another possible approach is to prepare handover on all Femtos with the same PSC. However it will be appreciated that such an approach is extremely wasteful of resources and may soon become unmanageable as the number of deployed femto base stations and user equipment increases.
Enterprise users tend to pay a premium to an operator in return for the provision of a higher quality of service compared to standard users. Hence any degradation to that service is potentially significant as it can negatively impact the enterprise user's perception of the value they are receiving from paying for that premium service.